Thermal IR sensors and methods of measuring IR radiation by means of semiconductor circuits, in particular by making use of MEMs structure suspended in a cavity, and comprising a temperature sensor, are well known in the art.
Most IR sensors transfer an incoming IR signal into a temperature increase of a thermally isolated structure (referred to herein as “absorber”) arranged for absorbing the IR radiation. The absorber is typically a so called membrane or diaphragm suspended in a sealed cavity by means of a suspension structure (e.g. long and thin beams). The more IR-power the isolated structure receives, the higher the temperature of the absorber will be with respect to the bulk (substrate and cap). For each amount of IR radiation, there is an equilibrium temperature at which the heating-up due to the incident IR power equals the heat loss from the absorber to the surrounding substrate and cap via heat conduction, heat convection and heat radiation. The temperature increase of the absorber is thus an indication of the amount of incident IR radiation, and is typically measured by means of a resistor with a high temperature dependence (bolometer) or by means of a series of thermocouples (thermopile). In this document the heat absorber with temperature sensor is referred to as a “pixel”.
For good sensitivity of the pixel, the temperature output signal would preferably be as large as possible for a given amount of IR-radiation power. The sensitivity of such a pixel is determined by three physically different gain factors: The first factor is determined by absorption and reflection of the IR light through the lid or cap onto the absorber. The second gain factor is determined by the thermal heat resistance between the absorber and the bulk of the device. This second factor especially depends on the heat resistance through the suspension structure (e.g. beams) of the thermally isolated structure and the heat resistance through the surrounding gas. The third factor is determined by the thermometer which is typically a resistor with high temperature dependence or a series of thermocouples (thermopile).
The stability and linearity of such a sensor clearly depends on the stability and linearity of the different gain factors, and often compromises have to be made between signal amplification and amplification stability. One of the most difficult parameters to control is the heat conduction from the absorber through the gas that surrounds the absorber. At atmospheric pressure (i.e. 1000 mbar) the heat conductance through the surrounding gas usually dominates the heat conductance through the suspension structure, whereas at vacuum (e.g. below 0.05 mbar) the heat transfer through the suspension (beams) dominates and the sensitivity is then maximized by minimizing heat transfer through the surrounding gas. For pressures ranging from about 0.01 to about 50 mbar the sensitivity (of a given pixel design) strongly depends on the gas pressure (as illustrated in FIG. 1). For this pressure range the sensitivity and stability of the sensor is largely dependent on the quality of the manufacturing process which determines the gas pressure and hermeticity of the sensor, in particular the step of bonding the cap to the substrate.
US2007069133 describes a microbolometer IR focal plane array that recognizes the problem of pressure variations in the cavity. It discloses that the pixel element is first used to measure the vacuum level (by a technique that uses active local heating, which is known in the art as a “Pirani measurement”), and once the vacuum level is determined, the measured signal due to the external IR radiation can then be compensated by means of calibration curves.